Posted
by
Soulskill
on Tuesday January 21, 2014 @01:36PM
from the cue-benny-hill-music dept.

sciencehabit writes "The European comet-chasing probe Rosetta is up and running again today after it successfully roused itself from a 2½-year sleep and signaled anxious controllers on the ground. The spacecraft had been put into hibernation during the most distant part of its 10-year journey in pursuit of comet 67 P/Churyumov-Gerasimenko because sunlight was too dim to keep its solar-powered systems running. Dozing in a slow stabilizing spin, Rosetta could not receive signals from the ground, so there was a risk that some problem might prevent it from responding to its preset alarm call at 10:00 GMT. Even then, there were many processes to go through before news reached Earth: The spacecraft's heaters would need to warm up its systems, its startrackers get a fix, boosters halt the spin, solar arrays turn towards the sun, and, finally, its communications antenna would need to point at Earth. It was not till 18:18 GMT that the signal was picked up by NASA's ground stations at Goldstone, California, and Canberra in Australia, and transmitted to the European Space Agency's (ESA's) control center at Darmstadt in Germany."

The spacecraft wasn't designed to operate that far out in space and it wasn't designed to handle the comet it's chasing. That anything about the mission is going well at all since they blew their initial launch window and had to retarget [spacedaily.com] is a miracle.

Oh, no, this is certainly a miracle. The miracle is that politicians actually budgeted sufficient money to carry out a program that wouldn't complete until after their term in office had ended. If Congress had been told that Opportunity's mission was going to last a decade the funding would never have been approved.

Yeah, I know, replying to myself. I can't help remembering Voyager's 'Grand Tour' to the outer planets. Congress refused to approve a mission of that extent, instead NASA had to package it as a much shorter mission to Jupiter and Saturn. They (rather sneakily, for a government bureaucracy) launched during the only window that would allow the Grand Tour, and then went after supplemental funding for the supposedly "extended mission" they had planned for all along. Still amazes me that the Shrub White House tried to cancel the miniscule cost of continuing to monitor the spacecraft.

Speed is relative, so is velocity. Rosetta is going to rendesvous with the comet, and go into orbit around it. At that point the speed and velocity will both be quite slow. I'm guessing that the biggest problem for the lander will be not bouncing off or floating away - there's next to no gravity.

Well, lack of atmosphere means that you need more propellant to equalize velocity. To land on a body with an atmosphere you have to just carry shielding and hit it at the right angle and the friction does the rest. The problem is that this gets you to terminal velocity and not zero velocity, and you don't want to hit the ground at terminal velocity.

If you're going to intercept a body without an atmosphere you have to equalize speed with only the use of propellant, so that is a lot more mass to carry. How

To land on a body with an atmosphere you have to just carry shielding and hit it at the right angle and the friction does the rest.

Except Mars has such an incredibly thin atmosphere that a parachute needs to be impossibly large for a soft landing. The gravity is too high for a rocket-powered landing like on the moon. Not to mention that same thin atmosphere being thick enough that you also need a tough heat shield.

No, that would be why they have to combine several of the above methods to even get a HARD landing. And yes, Mars has proven to be the most difficult body to land on... Only the US has managed it, and not at an impressive success rate, either.

To land on a body with an atmosphere you have to just carry shielding and hit it at the right angle and the friction does the rest.

Except Mars has such an incredibly thin atmosphere that a parachute needs to be impossibly large for a soft landing. The gravity is too high for a rocket-powered landing like on the moon. Not to mention that same thin atmosphere being thick enough that you also need a tough heat shield.

Actually, the atmosphere gets you 99% of the way. As I said in my post, an atmosphere only gets you to terminal velocity, so you usually still have some slowing down to do.

Compared to interplanetary velocity, terminal velocity is barely moving at all. On Mars it just happens to still be high enough to smash the probe. If you had to decelerate the probe completely to rest using only propellant (such as to land on one of Mars's moons) you'd need a lot more propellant. Actually, Mars's moons have the advan

It can, but it's not "free". You need a lot of heavy equipment to use that thin atmosphere to slow down. Landing on something without an atmosphere, and with low gravity, might only take a tiny fraction as much weight in fuel.

Compared to a planet, a comet is tiny. You essentially need a perfect intercept to minimize the delta-v required to enter comet orbit. Then they have to stick the landing and keep the probe from flying off the surface.

Gotta love KSP! I should look into modding in a tiny body like a comet just to test with. Someone on Reddit used Gilly as a comparable body for a "test run".

The most annoying thing about trying to land on a comet is that you can't timewarp past 1x anywhere close to it, and the gravity is so low it takes forever to actually land, or have your Kerbal come down after jumping.

(Would be cool if they'd add a couple to KSP. I bet there's a mod that does)

Here is a beautiful interactive 3D simulation of how Rosetta got to where it is now. Where is Rosetta? [esa.int]. Video [esa.int]
The choreography of the Earth, Mars, Earth, Earth slingshots is just amazing.
Here is the complex orbits to come of Rosetta around the comet Orbit around Comet [esa.int]

It took 8 hours and 18 minutes to warm up its systems, get a location fix, halt the spin, turn towards the sun, and, finally, point its communications antenna at Earth. Bah, I do that in 15 minutes *every* morning.

As I see it a Mars Outpost like the Outpost we have operated at the south pole for the last half century is possible in the foreseeable future (50-100 years), but a Mars Colony that did not require a constant lifeline of supplies just to survive is something best left to the sci-fi writers talking about the year 3,000.